1. Field of Invention
Because of its very high melting point of 2650.degree.C and its chemical inertness at high temperatures, pure zirconia appears, at first sight, to be very attractive as a material for the preparation of ceramics usable at very high temperatures but dense bodies of pure zirconia undergo a destructive volume change, which is accompanied by a change in structure from monoclinic to tetragonal, at about 1100.degree.C. It is known to reduce this volume change by mixing certain other oxides, such as calcia and magnesia, with zirconia prior to firing the mixture to convert the zirconia to a cubic modification, which is stable between room temperature and the melting point of the mixture. By restricting the additions of these other oxides to about 15 mole% with respect to the zirconia, the melting point of the zirconia is little affected and ceramic bodies of such a "stabilised" zirconia have become well known articles of commerce. However, it is difficult to prepare high density stabilised zirconia bodies when calcia is used as the stabiliser whilst bodies containing magnesia de-stabilise on thermal cycling in the temperature range 1000-1500.degree.C.
2. Description of Prior Art
Other oxides of the general formula R.sub.2 O.sub.3 are known to be capable of producing the desired cubic modification in the zirconia when added in a quantity of at least 6 mole% with respect to the zirconia. One of the first of these oxides to be discovered to give this effect and the one which has been the subject of the largest number of subsequent investigations is yttria, Y.sub.2 O.sub.3. These investigations have established that cubic stabilised zirconia containing 6 mole% Y.sub.2 O.sub.3 can be formed from a mixture of the oxides when heated to 2000.degree.C. These investigations indicated that oxides of other trivalent elements with a similar ionic radius to yttria, i.e. scandium and the rare earth metals from samarium (Atomic No. 62) to lutecium (Atomic No. 71) should also stabilise zirconia in the same modification when added in 6 mole% or greater amounts with respect to the zirconia. More recent investigations have shown that the addition of smaller quantities of yttria to zirconia, for example from 4 up to 6 mole% produce a tetragonal stabilised zirconia which does not undergo any phase transformation at elevated temperatures, that is temperatures ranging up to about 2000.degree.C.
Other developments have shown that ceramics based on yttria stabilised zirconia possess additional useful features. U.S. Pat. No. 3,432,314 describes the preparation of mixtures of very pure zirconia and yttria by controlled hydrolysis of their alkoxides and discloses that these mixtures can be sintered at temperatures as low as 1450.degree.C to give stabilised zirconia ceramics which have nearly theoretical density and which are translucent in thin sections. This United States specification also describes ytterbia and dysprosia stabilised zirconia using the same method of preparation.
It has been found that while yttria is similar to lime in permitting the production of stabilised zirconia ceramics which are resistant to de-stabilisation on thermal cycling, it has the additional advantages that the ceramics produced have superior corrosion resistance, for example, to molten glass, molten metals and titanates, and are better conductors of electricity at elevated temperatures than ceramics prepared from lime-stabilised zirconia.
It is clear that zirconia stabilised with yttria or one of the rare earth metal oxides ytterbia or dysprosia has considerable potential in the production of strong dense ceramics resistant both to temperatures in excess of 2000.degree.C and conditions of extreme corrosion. Pure yttria is a very expensive material since it usually occurs in association with rare earth metal oxides in ores such as monazite or xenotime from which it is only separated by lengthy and hence expensive procedures. Similarly, the rare earth metal oxides ytterbia and dysprosia are usually found in association with other rare earth metal oxides and to obtain them in the pure state is for the same reasons, expensive.
However, these separation procedures generate intermediate products containing 35% to 70% yttria with the remainder consisting substantially of oxides of rare earth metals whose atomic numbers range from 57 to 71.
The rare earth metals are commonly divided into two sub-groups, the cerium or "light" rare earth metal sub-group consisting of elements whose atomic numbers range from 57 to 61, and the yttrium or "heavy" rare earth metal sub-group consisting of rare earth metals whose atomic numbers range from 62 to 71.
It will be noted that yttrium, atomic number 39, is not, itself, a rare earth metal, although it occurs, in nature, in association with rare earth metals.